Dimerization of olefins using tantalum and niobium catalysts

ABSTRACT

Tantalum and niobium catalysts or catalyst precursors of the formula: ##STR1## wherein R is cyclopentadienyl, which can be substituted with methyl up to five positions, or neopentylidene, R 1  is neopentyl or benzyl, n is 0 or 1, R 2  is neopentylidene, benzylidene, tetramethylene, or 2,3-dimethyltetramethylene, A is halo or a moiety of the formula YR 3  R 4  R 5  wherein Y is a group 5 element including N, P, As, Sb and Bi and R 3 , R 4 , R 5  can be the same or different and are C 1  -C 4  alkyl, aralkyl such as naphthyl, neophyl, tolyl or xylyl, aryl such as benzyl or phenyl or bipyridyl and Z is tantalum or niobium. The compounds selectively dimerize 1-olefins to 1-butenes; m=1 or 2.

The government has rights in this invention pursuant to Gret No. CHE76-07410 and IPA-0010 awarded by the National Science Foundation.

REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 32, filedJan. 2, 1979, which is a continuation-in-part of application Ser. No.883,628, filed Mar. 6, 1978, and now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to catalysts or catalyst precursors for forming1-butenes from a 1-olefin reactant.

Prior to the present invention, there have been no known homogeneouscatalysts for converting a terminal olefin selectively to a 1-butene or2,3-disubstituted 1-butene at room temperature or above. Knownhomogeneous dimerization catalysts readily isomerize the initiallyformed 1-butene to the thermodynamically more stable internal butene. Itis also known that heterogeneous catalysts such as chromocene on aluminaor silica product 1-butene from ethylene. In addition, other selecteddimerization reactions are known. However, these reactions generally aresuccessful for only one olefin, e.g., the use of KC₈ to dimerizepropylene selectively to 4-methyl-1-pentene.

Accordingly, it would be desirable to provide a homogeneous catalystsystem for selectively forming 1-butenes which can be used with any oneof a plurality of 1-olefin feed compositions.

SUMMARY OF THE INVENTION

This invention provides a class of tantalum or niobium catalysts orcatalyst precursors for dimerizing 1-olefins to 1-butenes selectively.The catalysts are useful with C₂ to C₄ olefins and particularly withethylene to produce substantially pure 1-butene. The catalysts arehomogeneous in that they are in the same phase as the reactants underthe conditions utilized in the reaction.

The catalysts of this invention are represented by the formula: ##STR2##wherein R is cyclopentadienyl, C₅ H_(x) Me_(5-x), wherein x is aninteger from 0 to 5, or neopentylidene, R₁ is benzyl or neopentyl, n is0 or 1, R² is neopentylidene, benzylidene, tetramethylene or2,3-dimethyltetramethylene, A is halo including chloro, bromo, iodo andfluoro or a moiety of the formula YR³ R⁴ R⁵ wherein Y is a group 5element including N, P, Sb, and Bi and R³, R⁴, and R⁵ can be the same ordifferent and C₁ -C₄ alkyl, aralkyl, i.e. naphthyl, neophyl, toluyl orxylyl, aryl, i.e. benzyl or phenyl, or bipyridyl and Z is tantalum orniobium; m=1 or 2.

When A is YR³ R⁴ R⁵, the compounds of this invention are prepared inaccordance with the reaction I. (m=2) ##STR3## In accordance withreaction I, a tantalum catalyst of this invention is prepared asfollows: ##STR4## The reaction is conducted in an inert solvent such aspentane, benzene, decane or the like, at a temperature between about 0°and 150° C., preferably between about 20° and 40° C. Water and oxygenmust be excluded. An inert atmosphere is required. The product (B) canbe recovered by crystallization.

The reaction II can also be conducted in the presence of a C₂ -C₄ olefinsuch as ethylene. The product (B) reacts instantly with the olefin toform a substantially pure 1-olefin dimer which in the case of ethyleneis 1-butene. In this reaction, compound B functions as a precursor to acatalyst which effects the dimerization since, during the reaction, thetwo neopentylidene ligands and the neopentyl ligand are cleaved off. Thereaction can be conducted at atmospheric pressure or preferably at anelevated pressure, usually at a pressure between 45 and 60 psi. At theelevated temperatures and pressures, reaction rates are substantiallyincreased. The compound A or its niobium analog is made by the processset forth in U.S. Pat. No. 3,988,332 which is incorporated herein byreference.

The compounds of this invention wherein A is halo and R iscyclopentadienyl are prepared by the following general reaction whereinA is chloro(III). The same reaction scheme applies when A is bromo,fluoro, and/or iodo. ##STR5## wherein R⁶ is hydrogen, methyl or ethyl.The compounds (C), in the presence of a C₂ -C₄ olefin, catalyticallyeffect dimerization of the olefin to the corresponding 1-butene. Thereaction of compound D and the olefin is effected in a dry, oxygen-free,inert solvent such as pentane, benzene, decane or the like, at atemperature between about 0° and 200° C., preferably between about 20°and 100° C. and at a pressure between about 15 psi and 1500 psi,preferably between about 45 psi and 60 psi. The olefin dimer isrecovered by distillation after the reaction has been allowed to proceedto completion as indicated by gas chromatography.

The starting meterial (D) can be prepared by reacting either tantalumpentachloride or niobium pentachloride with Zn(CH₂ CMe₃)₂ at atemperature between about 20° and 40° C. in toluene or pentane to formthe compound: (Me₃ CCH₂)₂ ZCl₃. The latter compound then is reacted withTlC₅ Me_(x) H_(5-x) such as TlC₅ Me₅ or LiC₅ Me_(x) H_(5-x) such as LiC₅Me₅ wherein X is 0 to 5 at a temperature of between about 20° and 40° C.in toluene or diethyl ether to form the starting material (D).Representative compounds prepared by this invention include: ##STR6##

In the process of this invention, the following group 5 donors can beemployed: PMe₃, NMe₃, BiMe₃, PMe₂ Ph, AsMe₃, P(OMe)₃ or an opticallyactive phosphine of the formula PR¹ R² R³ when R¹ R² and R³ are definedabove.

The catalysts and catalyst precursors of this invention providesubstantial advantages over the prior art in that the reactants andcatalysts or catalyst precursors can enter the reaction in a singlephase and the products produced are substantially pure 1-olefin dimers,i.e., at least about 95 weight percent pure.

SPECIFIC EMBODIMENTS OF THE INVENTION

The following examples illustrate the present invention and are notintended to limit the same nor be construed as fully delineating thescope of this discovery.

In order to avoid the presence of molecular oxygen and moisture, allexperiments below were carried out in an atmosphere of dry nitrogen gas.

EXAMPLE I

This example illustrates the method for making the compounds of theformula ##STR7## wherein Z is Ta or Nb.

4.3 grams of Ta(CH₂ CMe₃)₂ Cl₃ (or 3.4 grams of Nb(CH₂ CMe₃)₂ Cl₃) wasdissolved in toluene and 2.4 grams of TlC₅ H₅ added and reactedtherewith at 25° C. for 24 hours. A residue, in each case, was obtainedafter filtering off TlCl and removing all solvent in vacuo. It wasrecrystallized from pentane/toluene (5:1) at -40° C. to give a 70 weightpercent yield of the Ta compound and 15 weight percent yield of the Nbcompound. (Calcd for TaC₁₀ H₁₅ Cl₂ : C, 31.03; H, 3.90; Cl, 18.32.Found: C, 31.17; H, 4.05; Cl, 18.30. Calcd for NbC₁₀ H₁₅ Cl₂ : C, 40.17;H, 5.05; Cl, 23.71. Found: C, 40.19; H, 5.16; Cl, 23.66.). The ¹ H and¹³ C NMR spectra for both compounds are entirely consistent withformulation as neopentylidene complexes. [δ¹³ C.sub.α =246 ppm (Z=Ta)downfield of internal tetramethylsilane standard in C₆ D₆ ; ¹J_(C).sbsb.α_(H) =84 Hz]. A molecular weight measurement suggests the Tacompound is a monomer in benzene. The Ta compound is thermally stable,unchanged after 3 h at 150° C. in benzene is a sealed tube orsublimation at 1μ and 65° C.), but reacts readily with moist air. As inother tantalum alkylidene complexes, the neopentylidene α-carbon atom isbelieved to be nucleophilic, e.g., it reacts with HCl at -78° C. intoluene to give Ta (η⁵ --C₅ H₅)Cl₃ (CH₂ CMe₃) in 95% isolated yield.

EXAMPLE II

This example illustrates the method for making the compounds of theformula ##STR8##

Ethylene (45 psi) reacted readily with 3.88 grams (10 m mol) Ta(η⁵ --C₅H₅)(CHCMe₃)Cl₂ partially dissolved in 20 ml pentane at 25° C. The redcolor first deepened, then lightened to yellow-orange in 3-5 min asorange crystals of (η⁵ --C₅ H₅)Cl₂ TaCH₂ CH₂ CH₂ CH₂ fell from solution.The product was isolated in 95% yield by filtration. (Calcd for TaC₉ H₁₃Cl₂ : C, 28.98; H, 3.51; Cl, 19.01. Found: C, 29.03; H, 3.63; Cl,19.17.)

The -40° C. 60 MHz ¹ H nmr spectrum of (η⁵ --C₅ H₅)Cl₂ TaCH₂ CH₂ CH₂ CH₂in toluene-d₈ under 1 atm of ethylene shows a η⁵ --C₅ H₅ resonance at τ4.60, a small peak for free, dissolved ethylene at τ 4.75, and two broadpeaks (ca. 20 Hz wide) and two sharper peaks (ca. 10 Hz wide) at τ 6.35,7.15, 7.95 and 8.10, respectively, of approximately equal area, and atotal area of eight protons vs. η⁵ --C₅ H₅. On warming to 40° C., thebroad pair coalesces with the sharper pair to give two peaks at τ 7.25and 7.60; the peak for free ethylene remains sharp and does not shiftappreciably.

The 67.89 MHz gated decoupled ¹³ C spectrum of (η⁵ --C₅ H₅)Cl₂ TaCH₂ CH₂CH₂ CH₂ in C₆ D₆ was measured at 25° C. The two triplet resonances dueto the two types of carbon atoms in the tetramethylene-metal ring arefound at 89.7 and 33.5 ppm downfield of tetramethylsilane with ¹ J_(CH)=123±2 and 126±2 Hz, respectively. The doublet (¹ J_(CH) =181 Hz) due toη⁵ --C₅ H₅ is found at 112.8 ppm.

(η⁵ --C₅ H₅)Cl₂ TaCH₂ CH₂ CH₂ CH₂ has also been characterized bychemical reactions. It reacts with Br₂ in diethyl ether at -78° C. togive 1,4-dibromobutane (0.83 per Ta) and 1,2-dibromoethane (0.18 perTa), and with CO (3 atm) in diethyl ether at -78° C. (followed bywarming to 25° C.) to give cyclopentanone (0.50 per Ta) and nearlyinsoluble TaCp(η⁵ --C₅ H₅)Cl₂ (CO)₂ (0.55 per Ta; ν_(co) (cm⁻¹)=2945 s,1962 s).

The organic product of this reaction is almost exclusively4,4-dimethyl-1-pentene (Table I). The yield of 3,3-dimethyl-1-butene,the metathesis product, is insignificant and the yield oftert-butylcyclopropane must be ≲5%.

Propylene (45 psi) reacts with Ta(θ⁵ --C₅ H₅)(CHCMe₃)Cl₂ in pentane at0° in two hours to give a precipitate of (η⁵ --C₅ H₅)Cl₂ TaCH₂CHMeCHMeCH₂ which was isolated by filtration in 75% yield. In theabsence of propylene this compound decomposes in the solid state or insolution (in pentane, for example) at ≳25° C.

The proton decoupled 22.63 MHz ¹³ C nmr spectrum of (η⁵ --C₅ H₅)Cl₂TaCH₂ CHMeCHMeCH₂ in toluene-d₈ at 7° C. shows singlet resonances at113.5 (η⁵ --C₅ H₅), 95.6 (C.sub.α), 49.7 (C.sub.β), and 24.7 (C.sub.γ);C.sub.α is a triplet (¹ J_(CH) =123±2 Hz), C.sub.β a doublet (¹ J_(CH)=126±2 Hz), and C.sub.γ a quartet (¹ J_(CH) =128±2 Hz) in the 67.89 MHzgated decoupled ¹³ C spectrum.

The major organic product of this reaction is 2,4,4-trimethyl-1-pentene(Table I).

The major organic product of the reaction of Ta(η⁵ --C₅ H₅)(CHCMe₃)Cl₂with styrene and cis-3-hexene results from an analogous, selectiveinsertion of neopentylidene into an olefinic C--H bond (Table I). Aqualitative comparison suggests that the rate of reaction of these fourolefins with the Ta compound decreases in the orderethylene>propylene>styrene>>cis-3-hexene. The fact thatβ,β-dimethylstyrene did not react appreciably under conditions wherecis-3-hexene was consumed is consistent with this general trend. Thereaction of the Nb compound with ethylene also gives4,4-dimethyl-1-pentene, but in lower yield.

It is believed that the olefin adds to the metal-neopentylidene doublebond to give one metallocyclobutane intermediate selectively (e.g., 1, 2and 3, Scheme I), the one which is consistent with the neopentylideneligand's nucleophilic properties. A specific hydrogen atom shift fromC.sub.β to the tertbutyl-substituted C.sub.α in 1, 2, and 3 is one meansof generating the observed product.

                  TABLE I                                                         ______________________________________                                        Products of the Reaction of the Ta Compound                                   and the Nb Compound with Olefins.sup.a                                        Z   Olefin         Products                                                   ______________________________________                                        Ta  CH.sub.2CH.sub.2 (45 psi)                                                                     ##STR9##                                                  Ta  CH.sub.3 CHCH.sub.2 (45 psi)                                                                  ##STR10##                                                 Ta  PhCHCH.sub.2 (3 mol in C.sub.6 D.sub.6)                                                       ##STR11##                                                 Ta  cis-3-Hexenes (neat, 120° C.)                                                          ##STR12##                                                 Nb  CH.sub.2CH.sub.2 (45 psi)                                                                     ##STR13##                                                 ______________________________________                                         .sup.a Solvent = mesitylene, pentane, or C.sub.6 D.sub.6 ; T =  25.degree     C. unless otherwise noted. Minor product yields were measured only for th     first example.                                                                .sup.b Identified by GLC yield and .sup.1 H NMR comparison (in C.sub.6        D.sub.6) with an authentic sample.                                            .sup.c GLC yield based on internal standard (a saturated hydrocarbon).        .sup.d Proposal based on .sup.1 H NMR comparison (in C.sub.6 D.sub.6) wit     the spectrum of transpropenylbenzene.                                         .sup.e Yield determined in C.sub.6 D.sub.6 by .sup.1 H NMR vs. internal       cyclohexane standard.                                                         .sup.f The mass and .sup.1 H NMR spectra are consistent with the major        olefin product being one of this type.                                        ##STR14##

EXAMPLE III

This example illustrates the method for making the compounds of theformula: ##STR15##

To 25 ml of pentane containing 4.6 grams of Ta(CH₂ CMe₃)₃ (CHCMe₃) wasadded 1.7 grams of PMe₃. After 24 hours, the pentane and excess PMe₃were removed in vacuo to give 5.4 grams of virtually pure Ta(CH₂CMe₃)(CHCMe₃)₂ (PMe₃)₂ as a yellow-orange residue. It may berecrystallized from pentane as orange crystals.

One mole per Ta of the organic product, neopentane, was identified andmeasured by gas chromatography.

The product is monomeric in cyclohexane (by cryoscopic measurement).

The 270 MHz ¹ H nmr spectrum of this product in C₆ D₆ shows three CMe₃peaks at τ 8.795, 8.803 and 8.844, a CH₂ triplet (J_(HP) =19 Hz) at τ9.39, two ═CH-- peaks at τ 7.92 and 3.07, and a PMe₃ triplet (J_(HP)=2.4 Hz) at 96 8.68. The 67.89 MHz gated decoupled ¹³ C spectrum in C₆D₆ shows the two ═CH-- carbon atoms at 274 and 246 ppm downfield ofSiMe₄ (¹ J_(CH) =95 Hz and 85 Hz, respectively), along withcharacteristic peaks for the other carbon atoms in this compound.

A similar reaction of 0.46 grams of Ta(CH₂ CMe₃)₃ (CHCMe₃) with 0.26grams of PMe₂ Ph in 10 ml of hexane at 60° C. for two daysgave Ta(CH₂CMe₃)(CHCMe₃)₂ (PMe₂ Ph)₂ in virtually quantitative yield. It waspurified by recrystallization from hexane. Its ¹ H and ¹³ C nmr spectrashow all of the important and essential features found in thecorresponding spectra of the PMe₃ compound described above.

The method of forming the corresponding Nb compounds is virtuallyidentical to that for Z=Ta except the reaction of L (PMe₃ of PMe₂ Ph)with freshly prepared Nb(CH₂ CMe₃)₃ (CHCMe₃) is complete within one hourat the most in each case. Nb(CH₂ CMe₃)₃ (CHCMe₃) is prepared by mixingNb(CH₂ CMe₃)₃ Cl₂ and two moles of LiCH₂ CMe₃ in pentane at -78° C.followed by warming to 0° C. for 15 min. Each is isolated as in therespective Ta cases and purified by recrystallization from pentane.Their ¹ H and ¹³ C nmr spectra show all the important and essentialfeatures found in the spectrum of Ta(CH₂ CMe₃)(CHCMe₃)₂ (PMc₃)₂ above.

EXAMPLE IV

This example illustrates the dimerization of ethylene to 1-butene by thecatalyst precursor Ta(CH₂ CMe₃)(CHCMe₃)₂ (PMe₃)₂.

A stirred solution of 0.20 grams of Ta(CH₂ CMe₃) (CHCMe₃)₂ (PMe₃)₂ in 20ml of pentane at 30° C. in a pressure bottle was treated with ethylenemaintained at a pressure of 60 psi. The starting compound rapidly reactsand the C₅ groups are cleaved off as neopentylethylene (identified bygc/mass spectroscopic comparison with authentic sample). The solutiontakes up ethylene steadily at the rate of approximately 1 mmole min⁻¹per mole of Ta and the solution's volume steadily increases. After 8hours, the volatiles were removed in vacuo. Analysis by gaschromatography showed (in addition to pentane solvent) primarily1-butene with <5% 2-butenes and only traces of higher boiling products.No additional higher boiling products were found by gas chromatographyin the catalytic reaction before transferring the volatiles.

The dimerization reaction will continue indefinitely if molecular oxygenand water are excluded. At higher temperatures (approximately 80° C.)and ethylene pressures (approximately 1500 psi) the rate increasesmarkedly. These conditions are preferred for maximum rate of productionof 1-butene.

EXAMPLE V

This example illustrates the dimerization of propylene to2,3-dimethyl-1-butene by (η⁵ --C₅ H₅)Cl₂ TaCH₂ CHMeCHMeCH₂.

Ta(η⁵ --C₅ H₅)(CHCMe₃)Cl₂ (0.075 grams) in 10 ml decane at 45° C. reactsrapidly with propylene (45 psi) to give 2,4,4-trimethyl-1-pentene (seeTable 1) and (η⁵ --C₅ H₅)Cl₂ TaCH₂ CHMeCHMe CH₂ stoichiometrically.Under these conditions, 2,3-dimethyl-1-butene forms catalytically; itcomprises 93% of the product mixture as determined by monitoring thecatalytic reaction by gas chromatography. A second primary product (5%of the mixture) is 2-methyl-1-pentene. A third primary product (2% ofthe mixture) is tetramethylene. The same products in the same amountsare formed catalytically starting with previously isolated (η⁵ --C₅H₅)Cl₂ TaCH₂ CHMeCHMeCH₂.

At 45° C., the 2,3-dimethyl-1-butene is formed at a rate of about twomols per Ta per hour. The rate can be increased markedly by increasingthe temperature and propylene pressure.

EXAMPLE VI

This example illustrates the dimerization of 1-butene by (η⁵ -C₅ H₅)Cl₂TaCH₂ CHEtCHEtCH₂.

Ta(η⁵ --C₅ H₅)(CHCMe₃)Cl₂ in 10 ml decane at 45° C. reacts with 1-butene(10 psi) to give the expected cleavage product,2-ethyl-4,4-dimethyl-1-pentene, and an active catalyst (presumablythermally unstable (η⁵ --C₅ H₅)Cl₂ TaCH₂ CHEtCHEtCH₂) for thedimerization of 1-butene to 2,3-diethyl-1-butene (identified by gc/massspectroscopey: parent ion 126, base peak 57). The production of2,3-diethyl-1-butene can be followed by gas chromatography. No otherbutenes were found.

EXAMPLE VII

This example illustrates the method for making the compound of theformula ##STR16##

To 7.1 grams of orange Ta(CH₂ C₆ H₅)₃ Cl₂ (prepared from TaCl₅ and 1.5Zn(CH₂ C₆ H₅)₂ in toluene) in tetrahydrofuran was added 2.1 grams ofLiC₅ Me₅. After stirring overnight, the solvent was removed in vacuo andthe residue was extracted with 50 ml benzene. Pentane (50 ml) was addedand the solution stood at room temperature for 1 day to give 4.8 grams(68%) of Ta(η⁵ --C₅ Me₅)(CH₂ C₆ H₅)(CHC₆ H₅)Cl as a red powder. ¹ H nmr(δ, C₅ D₆, 90 MHz, 35°): 1.82 (s, 15, CH₃), 2.26 (Ab quartet, 2, CH₂,J=12 and 7 Hz), 6.73 (s, 1, =CH), and 6.58-7.38 (m, 10, C₆ H₅). ¹ H nmr(δ, C₆ D₆, 270 MHz, 25°): 1.789 (s, CH₃), 2.257 (AB quartet, CH₂, J=11.7and 30.9 Hz), 6.706 (s, =CH), and 6.855-6.965, 7.126-7.213, and7.325-7.352 (m, C₆ H₅ and C₆ H₅ ').

¹³ C nmr (δ, C₆ D₆, gated ¹ H, 67.89 MHz, 25°): 11.4 (CH₃), 64.4 (1:1:1t, overlapping d from splitting by diasterotopic protons, CH₂), 115.6(s, C₅ (CH₃)₅), phenyl regin complex and suggestive of two inequivalentphenyl groups, 148.4 (s, C.sub.β of benzylidene), and 221.5 (d, =CH, ¹J_(CH;b) =85 Hz).

IR (cm⁻¹, Nujol/NaCl): 3050 s, br; 3010 s, shoulder; 1940 w, br; 1870 w,br; 1795 w, br; 1730 2, br; 1650 w, br; 1590 s, br; 1485 vs, sh; 1290 w,br; 1195 s, br; 1175 vw, shoulder; 1155 vw, shoulder; 1060 m, shoulder;1025 s, br; 930 w, sh; 900 w, br; 800 s, br; 755 vs, br; 730 m, sh,shoulder; 695 vs, sh.

EXAMPLE VIII

This example illustrates the method for preparing a compound of theformula ##STR17##

Pure Ta(CH₂ CMe₃)₂ Br₃ (2.70 g) was dissolved in 50 ml of ether and 0.70g of LiC₅ Me₅ added. The flask was covered with foil and stirred for 1h. The ether was removed in vacuo and the residue was extracted into 125ml of pentane. The extract was filtered and stood at -30° overnight togive 1.45 g of small red needles. Decreasing the volume afforded 0.35 gadditional product; total yield=1.80 g (56%). The compound is benzene isred by transmitted light but blue-purple by reflected light.

¹ H NMR (τ, C₆ D₆): 7.96 (s, 15, C₅ Me₅), 8.54 (s, 18, CH₂ CMe₃), 9.35(s, 4, CH₂ CMe₃).

TaCp"(CH₂ CMe₃)₂ Br₂ (0.80 g) was dissolved in minimal CDCl₃ and thesolution was stood in the dark for two days. The solvent was removed andthe residue was doubly recrystallized from minimal pentane at -30°;yield 0.40 g (57%). (C_(p) "=C₅ Me₅)

¹ H NMR (τ, C₆ D₆): 5.18 (s, 1, CHCMe₃), 7.88 (s, 15, C₅ Me₅) 8.83 (s,9, CHCMe₃). ⁻⁻ C NMR (ppm, C₆ D₆, ¹ H gated decoupled): 249 (d, CHCMe₃,¹ J_(CH) =77 Hz), 119 (s, C₅ Me₅), 48.2 (s, CHCMe₃), 33.2 (q, CHCMe₃, ¹J_(CH) =126 Hz), 13.2 (q, C₅ Me₅, ¹ J_(CH) =128 Hz).

I claim:
 1. The process of forming a 1-butene selectively whichcomprises contacting a C₂ to C₄ olefin with the catalyst of the formula##STR18## wherein R is cyclopentadienyl, C₅ H_(x) Me_(5-x), wherein x isan integer from 0 to 5, or neopentylidene, R¹ is neopentyl or benzyl, nis 0 or 1, R² is neopentylidene, benzylidene, tetramethylene or 2,3-dimethyltetramethylene, A is halo or a moiety of the formula YR³ R⁴ R⁵wherein Y is a group 5 element and R³, R⁴ and R⁵ can be the same ordifferent and are C₁ -C₄ alkyl, aralkyl, aryl or bipyridyl and Z istantalum or niobium: m=1 or
 2. 2. The process of claim 1 for forming a1-butene selectively wherein Z is tantalum.
 3. The process of claim 1for forming a 1-butene selectively wherein Z is niobium.
 4. The processof forming a 1-butene selectively which comprises contacting a C₂ to C₄olefin with the catalyst of the formula ##STR19##
 5. The process offorming a 1-butene selectively which comprises contacting a C₂ to C₄olefin with the catalyst of the formula ##STR20##
 6. The process offorming a 1-butene selectively which comprises contacting a C₂ to C₄olefin with the catalyst of the formula ##STR21##
 7. The process offorming a 1-butene selectively which comprises containing a C₂ to C₄olefin with the catalyst of the formula ##STR22##
 8. The process offorming a 1-butene selectively which comprises contacting a C₂ to C₄olefin with the catalyst of the formula ##STR23##
 9. The process offorming a 1-butene selectively which comprises contacting a C₂ ;l to C₄olefin with the catalyst of the formula ##STR24##
 10. The process offorming a 1-butene selectively which comprises contacting a C₂ to C₄olfin with the catalyst of the formula ##STR25##
 11. The process offorming a 1-butene selectively which comprises contacting a C₂ to C₄olefin with the catalyst of the formula ##STR26##
 12. The process offorming a 1-butene selectively which comprises contacting a C₂ to C₄olefin with the catalyst of the formula ##STR27##